Boron aluminum nitride diamond heterostructure

ABSTRACT

A heterostructure having a heterojunction comprising: a diamond layer; and a boron aluminum nitride (B (x) Al (1-x) N) layer disposed in contact with a surface of the diamond layer, where x is between 0 and 1.

TECHNICAL FIELD

This invention relates generally to heterojunction and more particularlyto diamond heterojunctions.

BACKGROUND

As is known in the art, a heterostructure is a semiconductor junctionhaving layers of dissimilar semiconductor material with unequal bandgapsand wherein carriers generated in one material fall into a quantum wellor channel layer provided by the other material. As is also known in theart, over the last decade there has been considerable effort to developsemiconductors devices having gallium nitride (GaN) based channel layerselectronics owing to GaN's high mobility, saturation velocity, breakdownfield, chemical and thermal stability, and large band gap. These factorslead to power densities 10× that of gallium arsenide (GaAs) baseddevices, and make GaN the primary candidate for many power electronicsapplications. However, as military and commercial applications demandever-higher power densities and operating temperatures, there becomes aneed to explore new material systems that could satisfy theserequirements. Diamond has the potential to be the material of choice forthe next generation of power devices.

Diamond is comparable to or better than GaN in almost every category.Specifically, its electron and hole mobilities, band gap, breakdownvoltage and thermal conductivity exceed that of GaN. In particular, thethermal conductivity of diamond (6-20 W cm⁻¹ ° C.⁻¹) is also noteworthy.At a typical output power density of 5 W/mm, the performance of GaNHEMTs is thermally degraded on current substrates even when grown on SiC(thermal conductivity of 3.6-4.9 W cm⁻¹ ° C.⁻¹ depending on polytype).However, the development of diamond based devices has been limited bythe difficulty in growing single crystal diamond films or substrates, bythe difficulty in growing n-type diamond, and the lack ofheterojunctions with two dimensional gas confinement (2D gas) for highelectron mobility transistor (HEMT) fabrication.

SUMMARY

In accordance with the invention, an aluminum nitride diamondheterojunction is provided.

In one embodiment, the aluminum nitride is doped.

In one embodiment, a boron aluminum nitride (B_((x))Al_((1-x))N) diamondheterojunction where x is between 0 and 1 is provided.

In one embodiment, a heterostructure is provided having a heterojunctioncomprising: a diamond layer; and a boron aluminum nitride(B_((x))Al_((1-x))N) layer disposed in contact with a surface of thediamond layer, where x is between 0 and 1.

In one embodiment, the surface of the diamond layer has a (111)crystallographic orientation.

In one embodiment, the AlN is hexagonal AlN.

In one embodiment, the Boron is alloyed into the AlN.

In one embodiment, a boron aluminum nitride (B_((x))Al_((1-x))N) diamondheterojunction where x is between 0 and 1 is provided wherein theB_((x))Al_((1-x))N is doped with donors to provide carriers to thediamond

In one embodiment, a boron aluminum nitride (B_((x))Al_((1-x))N) diamondheterojunction where x is between 0 and 1 is provided wherein thediamond is pulse doped to provide carriers.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional sketch of a B(x)Al(1−x)N/Diamond highelectron mobility transistor (HEMT) formed on a diamond substrate withtwo dimensional (2D) localizations of electrons in the diamond accordingto the invention;

FIG. 2 is a cross-sectional sketch of a B(x)Al(1−x)N/Diamond highelectron mobility transistor (HEMT) formed an AlN or SiC substrate with2D localizations of electrons in the diamond according to anotherembodiment of the invention;

FIG. 3 is a cross-sectional sketch of a B(x)Al(1−x)N/Diamond doubleheterostructure HEMT formed on a AlN or SiC substrate with 2Dlocalization of electrons in the diamond channel according to theinvention;

FIG. 4 is a cross-sectional sketch of a B(x)Al(1−x)N/Diamond doubleheterostructure HEMT formed on a diamond substrate with 2D localizationof electrons in a diamond channel according to the invention;

FIG. 5 is a cross-sectional sketch of an Inverted Diamond/B(x)Al(1−x)NHEMT formed on an AlN or SiC substrate, with 2D localization ofelectrons in the diamond at the Diamond/B(x)Al(1−x)N interface accordingto the invention;

FIG. 6 is a cross-sectional sketch of a B(x)Al(1−x)N/Diamond doubleheterostructure HEMT formed on a AlN or Silicon Carbide substrate withan additional AlN buffer layer and with 2D localization of electrons ina diamond channel according to the invention; and

FIG. 7 is a cross-sectional sketch of a recessed gateB(x)Al(1−x)N/Diamond MESFET formed on a diamond substrate according tothe invention.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring now to FIG. 1, a B_((x))Al_((1-x))N/Diamond heterojunctionstructure 10, here a high electron mobility transistor (HEMT), is shownformed on a diamond substrate 12 where x is between 0 and 1. Moreparticularly, a diamond buffer layer 14 is formed on the diamondsubstrate 12, here with the surface of the diamond substrate 12preferably having a (111) crystallographic orientation. AB_((x))Al_((1-x))N layer 16 is grown epitaxially using plasma molecularbeam epitaxy (MBE) on the surface of the diamond buffer layer preferablyhaving a (111) crystallographic orientation to form a heterojunctionwith the diamond buffer layer 14. Source and drain contacts 20, 22, herefor example Ti/Al/Pt/Au, are formed in ohmic contact with theB_((x))Al_((1-x))N layer and a gate contact 24, here for exampleNi/Pt/Au, is formed in Schottky contact with the B B_((x))Al_((1-x))Nlayer 14. Here the structure has two dimensional (2D) localizations ofelectrons 26 in the diamond buffer layer 14 (i.e., the diamond bufferlayer provides the channel layer which provides a quantum well for thecarriers in close proximity to the B(x)Al(1−x)N/heterojunction.

It should be noted that the relative values of the bandgaps for theB_((x))Al_((1-x))N/Diamond heterostructure 10 are useful in order tounderstand the HEMT structure of FIG. 1 where the 2D gas is localized inthe smaller bandgap diamond part of the device. However, more importantare the conduction and valence band offsets that occur at theB_((x))Al_((1-x))N/Diamond interface. For example, in an n-type HEMTstructure the conduction band offset localizes the carriers into a twodimensional gas (2D gas) of electrons that enhances the mobility of thedevice. Also, in the n-type structure, the valence band offset helps tosuppress gate leakage current due to impact ionization generation ofholes (important in power devices). For the p-type device the roles ofthe band offsets are reversed. The valence offset band would localizethe 2D gas of holes, while the conduction band offset would confineelectrons generated during impact ionization. Furthermore the size ofthe conduction (valence) band discontinuity partly determines theconcentration of free carriers in the 2-Dimensional gas and thusdirectly impacts the current of the device. For AlN/Diamondheterostructures (x=0) the conduction band offset is only ˜0.2 ev, whilethe valence band offset is about ˜0.53 ev. A p-type HEMT should bepossible, while an n-type HEMT would have low current due to the smallconduction band offset. Another concern, which is discussed later, isthat this conduction band offset could be reduced further (or eveneliminated) by strain induced by lattice mismatch between theB_((x))Al_((1-x))N and Diamond.

The conduction band and valence band discontinuities (and thus increasethe device current and power) are increased by alloying Boron into AlN.Although BN is an indirect semiconductor, it has a large direct bandgap.By alloying Boron into AlN, the ternary bandgap (which for small Boronconcentrations will be direct) will increase until the material becomesindirect. After the material becomes indirect, the bandgap will decreasewith additional boron incorporation (due to the small indirect k valleybandgap of BN). In table below, the band gaps of the differentB_((x))Al_((1-x))N valleys based on composition are calculated. Thedesirable direct bandgap is the Gamma Valley. On the left, the minimumband gaps by composition, but not including bowing are highlighted. Tothe right, the bowing band gaps are calculated assuming first a bowparameter of 1, and then a parameter of 3. These parameters were assumedbased on AlGaN (bow parameter of 1) and InAlN (bow parameter of 3)because the bow parameter for B_((x))Al_((1-x))N is not known. Below thetop set of data is another set of data. The difference between the twosets is that they assume different band gaps for the K valley in BNbecause the quoted range was 4.5-5.5 eV. The top set of data is 4.5 eV(worst case scenario), the bottom set is 5.5 eV (best case scenario).These calculations were based on the wurtzite crystal structure forB_((x))Al_((1-x))N. To summarize, the largest band gap is at 20-25% BNconcentration. Without including conduction band bowing, the maximumbandgap is ˜6.77 eV. By including conduction band bowing, the maximumvalues range from ˜6.2-6.6 eV. Therefore in most cases the bandgap andthe conduction and valence band discontinuities with diamond areincreased by alloying boron into AlN, increasing the current density andconfinement capability of the structure. If other crystal structures areused in these calculations, the numbers will be different but theconcepts put forward herein will be the same.

The band gaps of the different B(x)Al(1−x)N valleys based oncomposition. The best (bottom) and worst case scenario (top) arepresented. The compositions of boron blocked out are the approximatedirect to indirect valley cross over compositions.

Worst Case Scenario Assuming K Valley in BN only 4.5 eV NO ConductionAssuming A Assuming A Band Bowing Bow Factor 1 Bow Factor 3 DirectIndirect Direct Indirect Direct Indirect Valley Indirect Valley ValleyIndirect Valley Valley Indirect Valley B (X) Comp Gamma Valley K M-LGamma Valley K M-L Gamma Valley K M-L 1 8.5 4.5 6.6 8.5 4.5 6.6 8.5 4.56.6 0.95 8.385 4.635 6.615 8.3375 4.5875 6.5675 8.2425 4.4925 6.4725 0.98.27 4.77 6.63 8.18 4.68 6.54 8 4.5 6.36 0.85 8.155 4.905 6.645 8.02754.7775 6.5175 7.7725 4.5225 6.2625 0.8 8.04 5.04 6.66 7.88 4.88 6.5 7.564.56 6.18 0.75 7.925 5.175 6.675 7.7375 4.9875 6.4875 7.3625 4.61256.1125 0.7 7.81 5.31 6.69 7.6 5.1 6.48 7.18 4.68 6.06 0.65 7.695 5.4456.705 7.4675 5.2175 6.4775 7.0125 4.7625 6.0225 0.6 7.58 5.58 6.72 7.345.34 6.48 6.86 4.86 6 0.55 7.465 5.715 6.735 7.2175 5.4675 6.4875 6.72254.9725 5.9925 0.5 7.35 5.85 6.75 7.1 5.6 6.5 6.6 5.1 6 0.45 7.235 5.9856.765 6.9875 5.7375 6.5175 6.4925 5.2425 6.0225 0.4 7.12 6.12 6.78 6.885.88 6.54 6.4 5.4 6.06 0.35 7.005 6.255 6.795 6.7775 6.0275 6.56756.3225 5.5725 6.1125 0.3 6.89 6.39 6.81 6.68 6.18 6.6 6.26 5.76 6.180.25 6.775 6.525 6.825 6.5875 6.3375 6.6375 6.2125 5.9625 6.2625 0.26.66 6.66 6.84 6.5 6.5 6.68 6.18 6.18 6.36 0.15 6.545 6.795 6.855 6.41756.6675 6.7275 6.1625 6.4125 6.4725 0.1 6.43 6.93 6.87 6.34 6.84 6.786.16 6.66 6.6 0.05 6.315 7.065 6.885 6.2675 7.0175 6.8375 6.1725 6.92256.7425 0 6.2 7.2 6.9 6.2 7.2 6.9 6.2 7.2 6.9

Best Case Scenario Assuming K Valley in BN 5.5 eV NO Conduction AssumingA Assuming A Band Bowing Bow Factor 1 Bow Factor 3 Direct IndirectDirect Indirect Direct Indirect Valley Indirect Valley Valley IndirectValley Valley Indirect Valley B (X) Comp Gamma Valley K M-L Gamma ValleyK M-L Gamma Valley K M-L 1 8.5 5.5 6.6 8.5 5.5 6.6 8.5 5.5 6.6 0.958.385 5.585 6.615 8.3375 5.5375 6.5675 8.2425 5.4425 6.4725 0.9 8.275.67 6.63 8.18 5.58 6.54 8 5.4 6.36 0.85 8.155 5.755 6.645 8.0275 5.62756.5175 7.7725 5.3725 6.2625 0.8 8.04 5.84 6.66 7.88 5.68 6.5 7.56 5.366.18 0.75 7.925 5.925 6.675 7.7375 5.7375 6.4875 7.3625 5.3625 6.11250.7 7.81 6.01 6.69 7.6 5.8 6.48 7.18 5.38 6.06 0.65 7.695 6.095 6.7057.4675 5.8675 6.4775 7.0125 5.4125 6.0225 0.6 7.58 6.18 6.72 7.34 5.946.48 6.86 5.46 6 0.55 7.465 6.265 6.735 7.2175 6.0175 6.4875 6.72255.5225 5.9925 0.5 7.35 6.35 6.75 7.1 6.1 6.5 6.6 5.6 6 0.45 7.235 6.4356.765 6.9875 6.1875 6.5175 6.4925 5.6925 6.0225 0.4 7.12 6.52 6.78 6.886.28 6.54 6.4 5.8 6.06 0.35 7.005 6.605 6.795 6.7775 6.3775 6.56756.3225 5.9225 6.1125 0.3 6.89 6.69 6.81 6.68 6.48 6.6 6.26 6.06 6.180.25 6.775 6.775 6.825 6.5875 6.5875 6.6375 6.2125 6.2125 6.2625 0.26.66 6.86 6.84 6.5 6.7 6.68 6.18 6.38 6.36 0.15 6.545 6.945 6.855 6.41756.8175 6.7275 6.1625 6.5625 6.4725 0.1 6.43 7.03 6.87 6.34 6.94 6.786.16 6.76 6.6 0.05 6.315 7.115 6.885 6.2675 7.0675 6.8375 6.1725 6.97256.7425 0 6.2 7.2 6.9 6.2 7.2 6.9 6.2 7.2 6.9

The growth of a B_((x))Al_((1-x))N/Diamond heterostructure of hexagonalAlN on cubic (100) diamond would be problematic for several reasons. Inaddition to the large mismatch, growing hexagonal films on a cubicstructure will cause a significant number of defects at the AlN/Diamondinterface, degrading the HEMT device structure. Instead here the boronaluminum nitride (B_((x))Al_((1-x))N) is more favorably grown on (111)Diamond. There are several benefits of this. First, the orientation ofthe carbon atoms in diamond appears as a hexagonal lattice. Consequentlygrowth on a hexagonal material on a hexagonal substrate which wouldminimize defect formation at the critical interface ofB_((x))Al_((1-x))N/Diamond. Second, the effective diamond latticeconstant of the hexagonal net of carbon atoms becomes A/(square root of2)=2.52 Angstroms (where A is the lattice constant of diamond). Thislattice constant is less than B_((x))Al_((1-x))N. As a result,B_((x))Al_((1-x))N should experience biaxial compressive strain whengrown on (111) diamond. Compressive strain on the BAlN from the diamondsubstrate will have the effect of increasing the bandgap ofB_((x))Al_((1-x))N rather than decreasing it as biaxial tensile strainon B_((x))Al_((1-x))N would cause. Finally, it should be noted thatadding boron into AlN not only increases the bandgap, but also slightlyreduces the lattice mismatch with diamond by making the ternary latticeconstant smaller.

A final consideration in employing the (111) orientation is that AlN andconsequently BAlN exhibits a large piezoelectric effect and spontaneouspolarization. These properties have been exploited in GaN HEMTs toachieve high device currents without doping. The (111) orientationmaximizes the effects. Consequently another approach to overcomingdoping difficulties in the BAlN/diamond HEMT structure is to exploit thepiezoelectric effect and spontaneous polarization.

Now that the heterostructure has been established, the method ofproviding electrons and holes for n and p type HEMT devices must beestablished. In the literature, it has been shown that diamond can bedoped p-type by boron to very high levels (1*10¹⁹/cm³), however reliablen type doping for single crystal diamond has remained elusive. Since theB_((x))Al_((1-x))N can be doped with donors, here the B_((x))Al_((1-x))Nis doped n-type and used to provide carriers to the Diamond 2D gas inn-type HEMT device structures. Conversely (since B_((x))Al_((1-x))N isdifficult to be doped p-type) pulse doping of the diamond p-type is usedto provide carriers to the channel in some p-type device structures.Taking advantage of the piezo-electric effect of B_((x))Al_((1-x))Ngrown on certain crystal orientations of diamond should provide n-typeor p-type carries to the 2D gas diamond channel in this HEMT. Thisalternative method may reduce or eliminate the need for doping in thesestructures. This same piezoelectric effect is responsible for thecarries found in undoped AlGaN/GaN HEMT structures.

With the basic methodology established, several variations on thisB_((x))Al_((1-x))N/Diamond device structure are defined. Thesevariations are located below in FIGS. 2-7 (not drawn to scale). For thepurposes of discussion these devices are all assumed n-type, however,p-type devices from these structures are also possible. In FIGS. 1-7, itcan be seen that in addition to HEMTs fabricated on diamond substratesthat HEMT structures on substrates other than diamond. For examplegrowing diamond on AlN substrates, or AlN/SiC substrates. The primaryadvantage is that such substrates are available in large substrate sizesand are more economical than diamond.

Thus, FIG. 2 is a sketch of a B_((x))Al(1−x)N/Diamond heterojunctionstructure, here a high electron mobility transistor (HEMT), formed anAlN or SiC substrate 12′ with 2D localizations of electrons in thediamond buffer layer 14.

FIG. 3 is a sketch of a B_((x))Al_((1-x))N/Diamond doubleheterostructure HEMT formed on a AlN or SiC substrate 12′ with 2Dlocalization of electrons in the diamond channel, i.e., layer 14 whichis sandwiched between a pair of B_((x))Al_((1-x))N layers 16 a, 16 b.Here the B_((x))Al_((1-x))N layers 16 a, 16 b are either uniformlyn-type doped or n-type pulse doped.

FIG. 4 is a sketch of a B_((x))Al_((1-x))N/Diamond doubleheterostructure HEMT formed on a diamond substrate 12 with 2Dlocalization of electrons in a diamond channel, i.e., layer 14.

FIG. 5 is a sketch of an Inverted Diamond/B_((x))Al_((1-x))N HEMT formedon an AlN or SiC substrate 12′, with 2D localization of electrons in thediamond at the Diamond/B_((x))Al_(1-x))N interface. Here, theB_((x))Al(1−x)N layer 16 is epitaxially formed on the substrate 12′ andthe diamond channel, i.e., layer 14 is formed on the B_((x))Al(1−x)Nlayer 16. Also, the source and drain contacts are formed in ohmiccontact with the diamond channel, i.e., layer 14 and the gate contact isformed in Schottky contact with the diamond channel, i.e., layer 14.

FIG. 6 is a sketch of a B_((x))Al_((1-x))N/Diamond doubleheterostructure HEMT formed on a AlN or Silicon Carbide substrate 12′with an additional AlN buffer layer 30 formed on the substrate 12′ withthe lower B_((x))Al_((1-x))N layers 16 b formed on the AlN layer 30 andwith 2D localization of electrons in a diamond channel layer 14.

FIG. 7 is a sketch of a recessed gate B_((x))Al_((1-x))N/Diamond MESFETformed on a diamond substrate 12. Here, the gate contact 24 is formed inSchottky contact with the diamond buffer channel layer 14.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, other device structures shown in FIGS. 1-7 could also usesimilar substrates and AlN buffer layer to provide more examples of thistype. Accordingly, other embodiments are within the scope of thefollowing claims.

1. An aluminum nitride diamond heterojunction.
 2. The heterojunctionrecited in claim 1 wherein the aluminum nitride is doped.
 3. A boronaluminum nitride (B_((x))Al_((1-x))N) diamond heterojunction where x isbetween 0 and
 1. 4. A heterostructure comprising: a heterojunctioncomprising: a diamond layer; and a boron aluminum nitride(B_((x))Al_((1-x))N) layer disposed in contact with a surface of thediamond layer, where x is between 0 and
 1. 5. The structure recited inclaim 4 wherein the surface of the diamond layer has a (111)crystallographic orientation.
 6. The structure recited in claim 5wherein the AlN is hexagonal AlN.
 7. The structure recited in claim 4wherein the Boron is alloyed into the AlN.
 8. A boron aluminum nitride(B_((x))Al_((1-x))N) diamond heterojunction where x is between 0 and 1and wherein the B_((x))Al_((1-x))N is doped with donors to providecarriers to the diamond
 9. A boron aluminum nitride (B_((x))Al_((1-x))N)diamond heterojunction where x is between 0 and 1 and wherein thediamond is pulse doped to provide carriers.